The present invention relates to an electrode matrix with a hollow or fibrous structure made of metallized (and optionally subsequently removed plastic fibers), and a welded-on thin current-collector lug for batteries or galvanic cells, of the generic type disclosed by German patent documents DE-PS 36 32 351 and DE-PS 40 32 610.
The fibrous structure electrode described in German Patent Specification 36 32 351 is made of metallized plastic fibers and provided with a current-collector lug welded onto one side. The current-collector lug overlaps the electrode at the periphery on one side, the fibrous structure electrode being compressed in the vicinity of the overlap in such a way that the current-collector lug remains within the nominal thickness of the fibrous structure electrode. The current-collector lug is welded to the electrode by a plurality of material projections which are disposed in the overlap region and which extend in the direction of the electrode. The material projections pressed into the electrode still remain completely inside the electrode even at the point of their highest elevations after the welding.
The joint thus formed between the current-collector lug and the electrode matrix is generally sufficiently strong to withstand stresses encountered in the pulling direction of the current-collector lug attached to the terminal towards the electrode matrix loaded with active mass. For stresses applied transversely to the electrode matrix after it is filled with active mass (splitting off), or for continuous vibratory loadings, however, the strength is too low. Moreover, other more robust electrode matrices with mounted current-collector lugs are too expensive because of their complicated current-collector lug shape which must be prepared using special tools and working steps, or because of the large number of joints for maximum-load cells with many narrow electrodes per stack.
German Patent Specification 30 26 778 discloses an electrode matrix in which the current-collector lug has toothed projections which are pressed into the matrix. The current-collector is joined to the porous matrix by an electrolytic metal deposit, and remains inside the thickness extension of the electrode matrix. However, during vibration loading, the joint is very weak, and the projections may vibrate out of the electrode matrix again. Furthermore, cracks are formed during the penetration of the projections into the electrode matrix, which weakens the joint.
German Patent Specification 37 34 131 describes a current-collector lug which is divided into at least three tongues, alternately bent on one or both sides out of the plane of the lug, which is pushed over preferably compacted area of the fibrous structure electrode matrix and is attached to the latter by means of spot welds. In this case too, the strength of the joint under extreme stress is low.
German Patent Specification 31 42 091 discloses a method of producing a fibrous structure electrode having a reinforced periphery to which the current-collector lug is attached by rivets or welding or is pushed into the slotted periphery. The resulting joint suffers from the same disadvantages described above.
The object of the present invention, therefore, is to provide an electrode matrix having a hollow or fibrous structure and welded-on thin current-collector lug in which: no crack formation (or only a small crack formation) occurs in the electrode matrix in the vicinity of the welded joint; good contact is produced over the entire weld zone (particularly in the peripheral portions) of the electrode matrix; and the welded joint has a high strength not only for tensile stresses but also for stresses in the transverse direction. Electrode matrices according to the invention can be used, inter alia, in traction batteries and also in maintenance-free cells, such as in aerospace batteries.
This and other objects and advantages are achieved according to the invention, in which the thin metallic current-collector lug welded onto the electrode matrix has a rectangular cross section. In the vicinity of the lower periphery of the current-collector lug, material projections are impressed which are pierced in the center to form sharp edges so that the sheet-metal tabs forced out project almost at a right angle, or slightly away from the central perpendicular to the current-collector plane. The pierced material projections need not be arranged in a row, but must be in a region in which the current-collector lug and the electrode matrix overlap before welding.
As a rule, it is advantageous to increase the number of the pierced material projections towards the left and right periphery of the current-collector lug and near the center of its lower periphery, with a lower number of pierced material projections per unit length in the area in between. The edge of the electrode matrix, which advantageously has a thickness of between 0.3 mm and 5 mm, may be situated within a region between 2 mm and 5 mm below the current-collector lug before the welding operation. An overlap in the region between 3 mm and 5 mm is preferred. If the electrode matrix is too far below the end of the current-collector lug, there is a risk, after welding, that it will project out of the upper matrix plane, increasing the risk of short-circuit formation in the assembled cell. If, on the other hand, the electrode matrix is at too small a distance below the end of the current-collector lug according to the invention, an unsatisfactorily small weld zone is produced between the electrode matrix and the current-collector lug, so that the action of the pierced material projections cannot be effective.
At the beginning of the welding operation, as the upper and lower welding electrodes are brought together, the projecting sheet-metal tabs of the pierced material projections on the underside of the current-collector lug are the first to penetrate the lug neck of the electrode matrix which has not yet been compacted and impressed. As a result of the pressing operation of the welding electrodes, the electrode matrix zone in which the current-collector lug and the electrode matrix overlap, is compacted, and the sheet-metal tabs simultaneously key into the electrode matrix. In the subsequent compacting process, the sheet-metal tabs are twisted or bent over in the cavities and interstices of the electrode matrix and correspond to a positive or nonpositive joint after completion of the compacting process. Moreover, they also have an intimate contact with the fibers of the matrix electrode, some of which are torn and also twisted and pinched at these points.
In choosing the number of material projections, care should be taken. If the spacing between the individual projections is too small, the electrode matrix is severely weakened during the welding operation and may break off during high stressing immediately below or at the pierced row of material projections, as if at a perforation. It has been found that the pierced material projections are preferably arranged at a spacing of between 1 mm and 2.5 mm from the lower periphery of the current-collector lug, and at a spacing of between 1.5 mm and 2 mm from one another. Furthermore, the pierced material projections should end up below the reinforced periphery of the electrode matrix, which is mechanically more robust than the rest of the electrode matrix, so that they can penetrate the electrode matrix and can alter themselves more easily. In this connection, a value of approximately 1 mm has been found to be the most favorable spacing of the pierced material projections from the reinforced periphery of the electrode matrix. The welding then occurs primarily in all those regions in which the contact between the current-collector lug and the electrode matrix is the most favorable. This is the case at the specific points having the pierced material projections over the entire lower zone of the current-collector lug and in the region in which the welding electrode projects most and extends parallel to the lower face of the electrode matrix and also to the lower welding electrode.
By further shaping the welding electrode resting on the current collector lug, it is possible to achieve a constant thickness of approximately 50 to 60% of the full thickness of the electrode matrix in the parallel extending and most heavily pressed region of the compressed electrode matrix almost to the end of the current-collector lug in the direction of the lower side of the current-collector lug, with the periphery of the current-collector lug tapering to about 70% to 80% of the full thickness of the electrode matrix in a gradually curved transition (radius) to the left, downwards and to the right starting from the area of the current-collector lug which overlaps the electrode matrix. The taper of the pressed-in and welded-in current-collector lug in the three directions mentioned, over the periphery of the current-collector lug starts in the electrode matrix without any step until the normal height of the upper structural surface of the electrode matrix is reached. As a result of the smooth transition (that is, by avoiding abrupt steps), the electrode matrix is not too severely constricted even at the end of the current-collector lug. This avoids cracks and unduly small bearing cross sections in the electrode matrix, and results in an improved mechanical robustness of the welded joint both during tensile and during flexural stressing. In addition, it is advantageous to round off the corners of the current-collector lug during punching.
Metallized plastic-fiber matrices, in particular felts, needle-punched felts, nonwovens or the like, and hollow-fiberstructure electrodes such as are proposed, for example, in German patent document DE-PS 40 32 610, are used as electrodes matrices. Preferably, these materials should have a porosity (in the unprocessed state) between 50% and 98% and a weight per unit area of between 50 grams per square meter and 100 grams per square meter; their fibers should have a diameter of 0.4 dtex to 7.3 dtex and a length of 15 to 80 mm. The activation, metallization and reinforcement by electroplating are carried out by the convention techniques, with nickel and copper, in particular, being used as metallized coating on the fibers. In the case where nickel is used, the coating of the electrode matrix is preferably between 25 and 300 milligrams per square centimeter of the electroplated material. The plastic materials which are also suitable for textile fibers, for example polyolefins, polyamides, polyacrylonitrile and the like, are suitable as material for the fibers, provided they are resistant to the electrolyte or expellable from the metallic skin. At the periphery where the current-collector lug is to be mounted, the electrode matrices are preferably provided with a peripheral reinforcement obtained by a thicker metal coating on the fibers situated at that point.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.